This invention pertains to semiconductor devices and their manufacturing methods, in particular to rear surface incident-type light receptacle devices, which are useful for detection of ultraviolet and other short wavelength energy beams, and their manufacturing methods.
In image-pickup devices based on charge-coupled device (CCD) architecture, incident photons are received by an array of pixel units (CMOS capacitors) on a silicon substrate. Each pixel comprises an electrode (sometimes called a xe2x80x9cgatexe2x80x9d) mounted on the silicon substrate with an intervening thin dielectric layer. In the silicon beneath each electrode, a depletion region (xe2x80x9ccharge wellxe2x80x9d) is formed. As a photon having an energy greater than the energy gap of the respective MOS capacitor enters the depletion region of a pixel, if the photon is absorbed in the depletion region, then the photon produces an electron-hole pair. The electron stays within the charge well to contribute to a charge accumulated there over a defined time interval. After the time interval (i.e., periodically), the respective charge wells of the pixels are xe2x80x9cread outxe2x80x9d in a controlled manner to downstream processing electronics that convert the charge data into corresponding image data. Several conventional schemes have been devised for outputting the charges from the pixels. One scheme is termed the xe2x80x9cfull-frame transferxe2x80x9d (FFT) scheme. With FFT, the optics used to direct incident light to the light-detecting surface can be at maximal aperture.
In certain types of conventional FFT-type CCD arrays, a representative pixel includes an electrode (xe2x80x9cgatexe2x80x9d) situated over the respective depletion region (charge well) of the pixel. For a CCD array sensitive to visible light, the electrode usually is made of ITO (indium tin oxide) or other suitable material that is transparent to certain wavelengths of incident light. Specifically, xe2x80x9clong-wavelengthxe2x80x9d light (e.g., visible light), to which the electrode has a relatively low absorption coefficient, passes from an upstream direction through the electrode to the respective charge well. In each pixel, the electrode is situated proximally to the respective charge well (but separated from the charge well by the thin dielectric layer). The charge well accumulates charge from photons of the long-wavelength light transmitted through the electrode. The electrode has large respective absorption coefficients for light wavelengths that are relatively short (e.g., ultraviolet light) as well as for certain particulate radiation such as electrons. This absorption significantly reduces the sensitivity of the CCD array.
So-called xe2x80x9cback-side-incidencexe2x80x9d (BSI) CCD arrays have been proposed for detecting short-wavelength light and certain types of particulate radiation. In these arrays, with respect to each pixel, the light-incidence surface is on a xe2x80x9cbackxe2x80x9d or xe2x80x9crearxe2x80x9d surface of the silicon substrate, opposite a xe2x80x9cfrontxe2x80x9d surface on which the dielectric layer and the electrodes (gates) of the pixel are formed.
In BSI CCD arrays, as noted above, the substrate is usually silicon. The substrate typically is xe2x80x9cthinxe2x80x9d compared to the thickness of the silicon substrate of a conventional front-side-incidence CCD array (typically 300 xcexcm to 500 xcexcm). The xe2x80x9cthinxe2x80x9d substrate in a BSI CCD array has a thickness of approximately 10 xcexcm to 20 xcexcm. The substrate is thin in a BSI CCD because short-wavelength light has a relatively large absorption coefficient in silicon. Short-wavelength photons are absorbed near the incidence surface and converted to electron-hole pairs in the substrate proximal to the light-incidence surface. If the substrate were thicker, then the electron-hole pairs would recombine within the substrate before electrons could reach the respective depletion regions. This recombination substantially reduces the sensitivity of the device. Also, a thicker substrate in a BSI CCD array would tend to xe2x80x9cmixxe2x80x9d the electrons produced in various pixels, which would reduce the image resolution of the device.
The thin CCD substrate used in a BSI CCD array has low mechanical strength. To increase the strength, a xe2x80x9creinforcing substratexe2x80x9d (made of, e.g., silicon or glass) conventionally is bonded to the xe2x80x9cfrontxe2x80x9d surface of the CCD substrate (i.e., the surface on which the electrodes are formed). Such reinforcement prevents damage to the CCD substrate during various subsequent processing steps executed on the surface of the CCD substrate of the BSI CCD array, and facilitates handling during later fabrication processes (such as dicing).
According to conventional bonding methods, the reinforcing substrate is bonded to the CCD substrate using an adhesive having a post-cure hardness exceeding a certain hardness threshold. The adhesion is created by first applying a liquid silicate or other silicon oxide material to the reinforcing substrate, such as conventionally used when applying a layer of borophosphosilicate glass (BPSG) or spun-on glass (SOG). Then, an epoxy or other general-purpose resin adhesive is applied to adhere the CCD substrate to the silicon oxide surface of the reinforcing substrate.
Operationally, BSI CCD arrays must transfer photons efficiently from the light-incidence surface (back surface) of the CCD substrate to the respective depletion regions (charge wells) located on the front surface of the CCD substrate. To facilitate efficient diffusion of photons, ideally no crystal defects or metallic impurities should be present in the CCD substrate.
Conventional methods for fabricating BSI CCD arrays are directed to preventing formation of crystal defects and maintaining unwanted metallic impurities to insignificant concentrations in the CCD substrate. First, a semiconductor (silicon) xe2x80x9cfabricationxe2x80x9d substrate is prepared as a wafer on which the CCD substrate is formed subsequently by epitaxial growth. The fabrication substrate is formed with intrinsic gettering (IG) to minimize formation of crystal defects at least in a surficial layer of the fabrication substrate. The surficial layer is termed the xe2x80x9clow-defect layer,xe2x80x9d and its surface is termed the xe2x80x9clow-defect surface.xe2x80x9d The epitaxy is performed on the low-defect surface to form the CCD substrate. After forming the epitaxial layer, other layers and the pixel electrodes are formed on the epitaxial layer, followed by bonding of the reinforcing substrate. During a downstream step, the fabrication substrate is removed, such as by wet-etching, as discussed below.
Using the conventional process described above, it is difficult to obtain a BSI CCD array having a desired high sensitivity and resolution for short-wavelength light. There are several reasons for this difficulty.
First, as noted above, silicon oxide and a resin adhesive normally are used to bond the CCD substrate to the reinforcing substrate. Although silicon oxide adheres well to the reinforcing substrate, this bond is vulnerable to failure, resulting in the CCD substrate peeling away from the reinforcing substrate, especially during later fabrication steps (e.g., dicing). In addition, the surfaces to be bonded together must be mutually level during bonding. Consequently, the bonding process is complex inasmuch as it involves applying a silicon-based material (silicon oxide) onto a silicon-based reinforcing substrate, polishing the silicon-based material, applying the adhesive, and high-temperature processing to cure the adhesive.
Second, when using a general-purpose resin adhesive for bonding, stress arises in the adhesive interface after the adhesive is cured. The stress arises from differences in thermal-expansion coefficients of the substrates being bonded versus the adhesive itself. The stress is manifest as bending strain of the CCD substrate, causing operational problems such as disruptions in the transmission of photons through the CCD substrate to the respective diffusion layers and in the conduction of electrons from the diffusion layers to the respective electrodes. The stress also causes physical damage to the pixels.
Third, after forming the epitaxial layer on the low-defect surface of the fabrication substrate, the fabrication substrate is removed, usually by wet-etching. Wet-etching proceeds through the fabrication substrate to the low-defect layer. As etching progresses through the thickness of the fabrication substrate to the low-defect layer, crystal-defect locations tend to be etched more readily and more rapidly. Hence, when etching approaches near the low-defect layer, crystal-defect scars form in the low-defect layer. With further etching, these crystal-defect scars propagate to the low-defect surface and hence to the epitaxial CCD-substrate layer, ultimately creating crystal-defect scars on the front surface of the epitaxial CCD-substrate. Crystal-defect scars on the front surface of the CCD substrate scatter incident light and also serve as recombination sites for electron-hole pairs, causing reduced resolution and sensitivity.
To reduce propagation of crystal-defect scars as described above, mechanical polishing conceivably could be used instead of or in combination with wet-etching. With mechanical polishing, the crystal-defect areas and non-defective areas more likely would be polished at equal rates. Thus, polishing could produce a level polished surface with no crystal-defect scars. However, interaction of abrasive particles with the abraded surface during polishing and the presence of residual abrasive particles on the surface after polishing produce a type of crystal defect termed a xe2x80x9cfabrication-distortion layerxe2x80x9d on the non-defect surface and thus on the surface of the epitaxial CCD-substrate layer. The fabrication-distortion layer causes scattering of photons, in a manner similar to scattering caused by crystal-defect scars. The fabrication-distortion layer also creates recombination sites for electron-hole pairs, thereby reducing resolution and sensitivity.
Fourth, anomalies occur during wet-etching. Whereas a liquid etchant could be applied as a thin film to the surface of the fabrication substrate being etched, etching normally is simplified (and accelerated) by immersing the entire fabrication substrate in the etchant. But, total immersion also would etch the reinforcing substrate, thereby weakening the entire structure. Furthermore, if etching of the reinforcing substrate is non-uniform, then a non-uniform surface is presented for attachment to the device package at the time the BSI CCD device is placed in a suitable package. This uneven-surface mounting also causes the light-incidence surface to be misaligned relative to incoming light to be detected, causing reduction in resolution and sensitivity of the CCD.
In view of the shortcomings of conventional methods and devices as summarized above, an object of this invention is to provide improved fabrication processes for manufacturing semiconductor devices, wherein during fabrication less stress is applied to a substrate upon which one or more elements of the semiconductor device are formed. Another object is to provide improved fabrication processes for manufacturing back-surface-incidence (BSI) CCD light sensors, wherein during fabrication less stress is applied to a relatively thin device substrate (xe2x80x9cCCD substratexe2x80x9d) upon which one or more light-sensing elements of the BSI CCD light sensor are formed. These processes include attaching a reinforcing substrate using an adhesive.
Yet another object is to provide semiconductor-device fabrication methods that substantially reduce, compared to conventional methods, incidence of crystal defects in the CCD substrate.
Yet another object is to provide semiconductor-device fabrication methods including steps of epitaxial growth of a device substrate on a low-defect surface of a fabrication substrate, bonding a reinforcing substrate to the device substrate, and protecting the reinforcing substrate so that it is not etched during a wet-etching step performed to remove the fabrication substrate.
To any of such ends, and according to a first aspect of the invention, processes are provided for fabricating a semiconductor device including a reinforcing substrate. In an embodiment of such a method, a semiconductor-device element is formed on an upper surface of a semiconductor substrate. An uncured resin adhesive is provided that is capable of bonding the semiconductor substrate to a reinforcing substrate. The resin adhesive is formulated to cure to a hardness of no greater than 40 as measured using the JIS-A hardness standard. A layer of the uncured resin adhesive is applied to the upper surface of the semiconductor substrate. The reinforcing substrate is applied to the layer of uncured resin adhesive, and the resin adhesive is cured. The reinforcing substrate can be, for example, silicon or glass. This embodiment reduces stress on the device substrate that otherwise would arise after curing of the resin adhesive. Stress is alleviated even if there is a significant difference in the thermal-expansion coefficients of the cured resin adhesive and the device substrate. As a result, bending and damage to the device substrate do not occur.
The uncured resin adhesive desirably has a pre-cure viscosity of no greater than 9 Ns/mL. This allows the uncured adhesive to be applied by spin coating, wherein the semiconductor substrate is rotated while the uncured resin adhesive is applied onto the upper surface of the semiconductor substrate. Spin-coating in this manner results in a uniform application of the adhesive.
Another embodiment of a method according to the invention is directed to a process for fabricating a back-surface-incidence (BSI) CCD light sensor. In this method, a semiconductor substrate is provided having a thickness of 10 xcexcm to 20 xcexcm. A light-receiving element is formed on the front surface of the semiconductor substrate, wherein the rear surface is destined to be a light-incidence surface of the light sensor. A layer of an uncured resin adhesive as summarized above is applied to the rear surface of the semiconductor substrate, over the light-receiving element. The reinforcing substrate is applied to the layer of uncured resin adhesive, followed by curing of the resin adhesive. Upon executing a process according to this embodiment, a BSI CCD light sensor is produced that exhibits no excessive stress imposed by the cured resin adhesive on the device substrate. Consequently, there is no bending or substrate damage at locations where the light-receiving element(s) are formed, resulting in consistent properties from one element to the next.
In yet another embodiment of a fabrication process according to the invention, a semiconductor fabrication substrate is produced with intrinsic gettering. A low-defect layer is formed on the front surface and rear surface of the fabrication substrate, with an intervening high-defect layer situated between the low-defect layers. An epitaxial layer is formed on the front-surface low-defect layer. A semiconductor-device element is formed on the front surface of the epitaxial layer. A layer of an uncured resin adhesive (such as summarized above) is applied to the front surface of the epitaxial layer and to the semiconductor-device element. A reinforcing substrate is applied to the layer of uncured adhesive, the adhesive is cured to bond the reinforcing substrate to the epitaxial layer. The rear-surface low-defect layer and the high-defect layer are removed by mechanical abrasion, and the front-surface low-defect layer is removed by wet-etching. In this process, the step of forming the semiconductor-device element can include forming a light-receiving element on the epitaxial layer, wherein the rear surface of the epitaxial layer is destined to be a light-incidence surface for the light-receiving element. Using this process, no crystal-defect scars form or remain on the epitaxial layer or on the front-surface low-defect layer. This results in a high-quality device substrate in which crystal defects are reduced substantially.
In yet another embodiment of a fabrication process according to the invention, a semiconductor fabrication substrate is produced with intrinsic gettering, as summarized above. A low-defect layer is formed on the front surface and rear surface of the fabrication substrate, with an intervening high-defect layer situated between the low-defect layers. An epitaxial layer is formed on the front surface of the fabrication substrate. A semiconductor-device element is formed on the front surface of the epitaxial layer. A layer of an uncured resin adhesive is applied to the front surface of the epitaxial layer and to the semiconductor-device element. A reinforcing substrate is placed on the layer of uncured adhesive, wherein the rear surface of the reinforcing substrate contacts the adhesive. The resin adhesive is cured, and a protective substrate is attached to the front surface of the reinforcing substrate, using a removable adhesive. The rear-surface low-defect layer, the high-defect layer, and at least a portion of the front-surface low-defect layer are removed by wet-etching, followed by removal of the protective substrate. During wet-etching, the protective substrate protects the reinforcing substrate from unintended etching, preventing the epitaxial layer from being misaligned when the device (with reinforcing substrate) is mounted in a suitable package. In this process, the step of forming a semiconductor-device element on the front surface of the epitaxial layer can include forming a light-receiving element on the epitaxial layer, wherein the rear surface of the epitaxial layer is destined to be a light-incidence surface for the light-receiving element. Hence, the process can be used to fabricate a BSI CCD light sensor.
In the method summarized above, the epitaxial layer desirably has a defect density of no greater than 1xc3x97103/cm3, as measured using the Secco etching method. An exemplary removable adhesive is hot wax, which can be washed off easily using fuming sulfuric acid, for example.
According to another aspect of the invention, any of various semiconductor devices are provided, as fabricated using any process within the scope of the invention. Such devices encompass BSI CCD light sensors.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.